Article I of the United States Constitution is the longest Article in the document. Its job is to, sometimes roughly and sometimes specifically, outline the size, responsibilities and rights of Congress. It borrows from the Articles of Confederation in some ways, such as with the word Congress. However, it only takes that which is thought best from that document and from the practices of the 13 original state governments. In this way, the Constitution establishes Congress more fully as one of three core governing bodies.
View the full text of Article I of the U.S. Constitution here.
Congress as defined by Article I is bi-cameral. Two houses make up the one legislature, much like the House of Lords and House of Commons in Parliament, but without the monarchy. The Senate is one of these bodies. According to the Constitution, the Senate is to have two Senators from each state. This is a fixed amount, so the Senate only grows as more states are added to the country. The terms of Senators are divided into thirds. One-third will serve 6-year terms, one-third will serve 4-year terms and the remainder will serve 2-year terms. This was only in effect during 1st Congress and when new states were added to the Union so that about 1/3 of Senate seats would be up for filling every 2 years.
The House of Representatives is populated differently from the Senate and it is a bit more complicated. The seats in the house are to be filled by people who were elected by the people. Some may say, "Well, that is how the Senate is filled too," but that was not the case at the founding. The Senate was to be chosen by the state legislature of each Senator.
The number of people in the Senate is determined by a census conducted every 10 years. Non-free individuals, in so many words, count for 3/5 of a vote each or in other words, 3/5 of non-free people count as people according to the Constitution. This controversial Three-Fifths Compromise swayed slave states on the fence about the document. There were never to be less than one Representative for each state and the number would never exceed 1 for every 30,000 people. New representatives are chosen every two years. There was initially some fear that the number would not be representative enough if the House fell to just one per state, but the House has only grown since the founding.
There are some qualifications members of Congress must meet before being eligible for election. Senators must be 30 years old and Representatives are to be at least 25 years old. This is ostensibly to prevent only people born with famous names or influence from being members of Congress. There are noticeably no religious or property qualifications, contrary to what the far, far right might imply. Members of the House must be citizens of the U.S. for at least seven years prior to election and Senators must be for nine years.
The rights, responsibilities and positions within Congress are enumerated within Article I. Examples include having to keep a journal of minutes, how to elect officers and the fact that Senate is to be paid by the government. The Article also limits states' abilities to do certain things, namely deal with foreign governments, without the consent of Congress or at all. Some of the details have changed over the centuries that have passed, but the shape of Congress, inspired in some ways by Parliament and existing state legislatures, has never changed.